Kaposi's Sarcoma (KS) is the most frequent neoplastic complication seen in patients who are infected with HIV and is especially prevalent in male homosexual AIDS patients (26, 66). It is generally thought to be associated with a sexually and blood borne agent that is transmitted independently of HIV (10). A large proportion of Kaposi's sarcoma (KS) lesions, from both HIV-associated AIDS patients and from non-HIV associated classical and endemic sources, have been found to contain two small DNA fragments representing sequences from a putative novel gamma class herpesvirus genome referred to as KSHV or human herpesvirus-8 (HHV-8) (6, 19, 45). Several additional rare lymphomas, including multicentric Castleman's Disease and Body Cavity Based Large Cell lymphomas (BCBL), but few other tumor or normal tissue samples contain these same DNA sequences (16-18, 57). Additional recent evidence has shown that these sequences are part of a large episomal viral genome in BCBL cell lines (46, 52), that HHV-8 DNA positive BCBL cell lines can be induced to produce herpesvirus-like particles (52), and that sera from most patients with KS (but not normal sera) have antibodies to HHV-8 proteins (30, 37). In addition, evidence has been found that HHV-8 is also associated with Multiple myeloma. (Rettig et al., Science 276:1851-1854, 1997.) Together, these results suggest the strong likelihood that this proposed new infectious virus may be the etiological agent of KS (13, 44, 46, 52).
The two small DNA segments found in the original KS lesion studied (19) represent parts of the genes encoding the ORF25 major capsid, the ORF26 minor capsid protein and the ORF75 protein, based on the nomenclature used for the most closely related gamma herpesvirus HVS. The proposed HHV-8 protein fragments of ORF26 and ORF75 display 60% and 30% amino acid identity with their HVS (2, 12) and EHV-2 counterparts (29) and show a slightly more distant relationship to EBV. The known gamma-1 herpesviruses include human EBV and its close relatives in great apes and old world primates (35). The gamma-2 viruses originally included the rhadinoviruses, exemplified by HVS and its close relatives in new world primates, and the cottontail rabbit virus H.sylvilagis. The biological feature of lymphotrophism, including the ability to immortalize and establish a circular plasmid or episomal latent state, was the major common feature that placed them all into the gamma herpesvirus group, and their preferences for B-cells (EBV) or T-cells (HVS) was used as a criterion to discriminate between the gamma-1 and gamma-2 class subgroupings (11, 43, 56). Subsequently, DNA sequence analyses of small segments of EHV-2 and EHV-5 (14, 67), bovine herpesvirus type 4 (BHV-4) and the mouse herpesvirus MHV68, have also revealed closer protein sequence relationships and gene order to HVS than to any other known herpesviruses, despite exhibiting a much broader permissive host cell range including cultured fibroblasts, and a latency trophism that may include T-cells, B-cells or macrophages (15, 25, 39, 60, 67).
Complete genomic DNA sequence data is now available for three prototype gamma herpesviruses including EBV (7), HVS (3, 4, 48) and EHV-2 (61). EHV-2 appears to represent a distinct subgroup of the gamma-2 herpesviruses based on its overall (G+C)-content of 58%, compared to the much lower 35% and 43% for the unique regions of HVS and BHV4 (14, 22), together with the presence of large 18-kb terminal direct-repeat structures, rather than the short multi-copy tandem-repeat patterns found at the termini of HVS and BHV-4 (9, 15, 58). Apart from the essentially co-linear organization of major gene blocks (31), each of these gamma herpesviruses contain several clusters of additional genes that are unique to each species and are often not found in any other herpesvirus (32). In EBV, these species-specific or subtype-specific genes include the EBNAs and all other latency-associated genes, the lytic cycle transcriptional and replication control protein ZTA, virus cell-type-specific receptors, and gene products that interfere with or protect against immune surveillance mechanisms (23, 32, 34, 41, 42). All of these optional genes, plus others with as yet unidentified functions, map to within one of six divergent loci in gamma herpesvirus genomes that we refer to from left to right as DL-A to DL-F (39). Amongst the relevant virus-specific and subtype-specific genes that are unique to gamma herpesviruses there is a thymidylate synthase (TS) gene in HVS and EHV-2, but not in EBV or BHV-4 (12, 36, 61). Both EBV and EHV-2, but not HVS or EHV-4, also encode vIL10 genes (54, 61, 65), and amongst the four only HVS encodes a DHFR gene (62). EBV and HVS, but not EHV-2 encode a Bcl-2 homologue and only HVS encodes a vIL17 gene (68). The TS, DHRF, vIL10 and vIL17 genes are highly conserved with the corresponding cellular versions (70 to 85% amino acid identity) and have been suggested to be relatively recent acquisitions as unspliced cDNA versions from the host mammalian genome.
Many intriguing questions arise as to how HHV-8 fits into this classification scheme and what is its evolutionary relationship to the other gamma herpesviruses, as well as whether HHV-8 is indeed an authentic infectious and ubiquitous human virus, rather than a relatively rare pathogen or recent acquisition from some exogenous primate source that has been spread more rapidly within the AIDS epidemic.
Thus there is a need in the art for additional tools for characterizing, diagnosing, and treating HHV-8 which has been found to be associated with a number of human diseases.